Existing prone breast biopsy systems such as described in U.S. Pat. No. 5,078,142 to Siczek et al. and U.S. Pat. No. 5,289,520 to Pellegrino et al. allow stereotactic needle biopsy of a breast, but with a limited field of view, e.g., typically a field of view of only 5 cm×5 cm. In cases where poorly visualized microcalcifications are detected on a screening mammogram, or as a result of diagnostic mammography, it is difficult to position that portion of the breast in the small field of view provided by the prone biopsy system, given the subtle nature of the microcalcifications and the lack of well defined landmarks in the breast. In addition, microcalcifications frequently involve one or more quadrants of the breast and sampling tissue from several areas is challenging for the radiologist and extends procedure time as the breast needs to be constantly repositioned and recompressed for additional imaging and biopsy.
Mammography and ultrasound are commonly employed to determine the extent of cancer present in a patient's breast once a known tumor has been diagnosed by ultrasound or stereotactic x-ray needle biopsy. However the sensitivity of mammography and ultrasound together falls short of the sensitivity provided by magnetic resonance imaging (MRI) with contrast.
Contrast MRI is extremely sensitive to the presence of breast cancer (>95% sensitivity), although MRI specificity is reported significantly lower at 30% to 60%. MRI imaging exams of one or both breasts are increasingly requested by radiologists and surgeons for patients with a known breast cancer, in order to determine the extent of disease. Should multifocality be detected (i.e., cancer present in more than one area of the breast) or if cancer is detected in the contralateral breast, surgical treatment may be more extensive, up to and including bilateral mastectomy. On the other hand, should no additional disease be detected by the MRI exam, a minimal surgical procedure such as a lumpectomy may be the preferred form of treatment. In addition to determining local extent of breast cancer (“local staging”) MRI breast imaging can also be employed to search for small cancers in asymptomatic women where cancer may be expected to develop.
In relation to the foregoing, several multi-institutional studies have now shown that MRI is an effective method of screening women who are at risk for breast cancer. See, e.g., Lehman CD et al., “Cancer Yield of Mammography, MR and U.S. in High-Risk Women: Prospective Multi-Institution Breast Cancer Screening Study,” Radiology, (August 2007); Kreige M. et al., “Efficacy of MRI and Mammography for Breast-Cancer Screening in Women with a Familiar or Genetic Predisposition,” New England Journal of Medicine, (Jul. 29, 2004); and Leach M. et al., “MRI Surveillance for Hereditary Breast Cancer Risk,” The Lancet, Volume 365 at 1769-1778 (2005). The American Cancer Society has recently adopted guidelines for annual MRI breast screening for women who have a lifetime risk of 20-25%. See, e.g., Saslow D. et al., “American Cancer Society Guidelines for Breast Screening with MRI as an Adjunction to Mammography,” CAA Cancer Journal for Clinicians, Volume 57 at 75-89 (2007). As many as 1.2 million women could be considered at high risk for breast cancer based on the guidelines, suggesting a need for significant MRI capacity for annual breast cancer screening.
In addition to screening for breast cancer with MRI, and as noted above, a role has emerged for MRI imaging of the contralateral breast when a breast cancer is diagnosed. Several studies have shown an incidence ranging from 3 to 18%. See, e.g., Lehman, “MRI Evaluation of the Contralateral Breast in Women with Recently Diagnosed Breast Cancer,” New England Journal of Medicine, Volume 356 at 1295-1303 (2007); Pediconi F., “Contrast-Enhanced MR Mammography for Evaluation of the Contralateral Breast in Patients with Diagnosed Unilateral Breast Cancer or High-Risk Lesions,” Radiology, Volume 243: Number 3 (June 2007). About 250,000 breast cancers are found annually in the United States (invasive and DCIS) suggesting that a cost effective procedure to rule out cancer in the contralateral breast will likely become a standard of care.
A significant limitation with MRI imaging of the breast is the difficulty of acquiring tissue using needle biopsy techniques. In this regard, an MRI guided breast biopsy is more difficult and time consuming than an ultrasound or stereotactic x-ray guided needle biopsy due to instrument requirements, and limited breast access.
That is, in MRI procedures the patient is normally positioned in the bore of a magnetic resonance MR scanner in a strong magnetic field that requires special non-metallic biopsy instruments capable of functioning within a strong magnetic field. In addition, since MRI patients are imaged in a prone position with the breast hanging pendulant within a breast coil, access to the medial portion of the breast is difficult as is access to tissue near the chest wall since most needle biopsy solutions for MRI systems provide only a lateral approach with a Cartesian grid location system, which by design necessarily inhibits needle access to tissue adjacent to the chest wall.
In addition to the foregoing, since the specificity of MRI in breast imaging is not as high as the sensitivity, it is difficult for many radiologists to confidently interpret an enhancing lesion of the breast with MRI without the performance of a difficult and time consuming needle biopsy procedure. As such, some physicians prefer not to order an MRI exam unless biopsy of all enhancing areas can be undertaken in order to provide the patient assurance that these areas of enhancement were indeed benign. In turn, MRI enhancement in a patient known to have breast cancer can create more uncertainty than if the exam had not been performed.
Finally, MRI imaging is also expensive compared to ultrasound or mammography and the procedure is not normally available in a breast center for near immediate scheduling such as other breast imaging procedures. For the noted reasons, alternatives to MRI breast imaging are of high interest.
Some imaging centers have experimented with contrast injection in conjunction with conventional multi detector CT (MDCT) imaging. The published results appear to be very similar to MRI imaging of the breast with contrast. See, e.g., Inoue et al., “Dynamic Multidetector CT of Breast Tumors: Diagnostic Features and Comparison with Conventional Techniques,” American Journal of Roentgenology, (September 2003); Tozaki et al., “Diagnosis of Tis/T1 Breast Cancer Extent by Muftislice Helical CT: A Novel Classification of Tumor Distribution,” Radiation Medicine, Volume 21: No. 5, at 187-192, (2003). However, conventional CT breast imaging techniques subject patients to radiation levels that are higher than desired due to the classical design of the axial CT scanner which necessarily images the entire thorax (i.e., lungs and heart) in order to include the breasts in the imaging field. In addition, state of the art conventional multidetector CT scanners (MDCT) are limited to providing spatial resolution of about 1 lp/mm. Further, this resolution may be achieved only with high resolution kernels (e.g., bone kernel) at the expense of increased image noise.
The ability to diagnose breast cancer based on morphologic imaging depends both on the uptake of contrast material into the cancerous tissue as well as the spatial frequency of the image, since breast cancer is frequently indicated by thin straight lines (i.e., spiculation) emanating from a lesion. Benign masses such as fibroadenomas are normally characterized by smooth oval shapes which may also take up contrast. MRI imaging of the breast allows a three-dimensional review of the entire breast with capabilities such as MIP (maximum intensity projection) and with MR pulse sequences which suppress signals from fat (fat suppression imaging) to improve the conspicuity of the contrast in the tumor and parenchymal tissue.
Prototype prone breast x-ray imaging systems that use cone beam CT in conjunction with commercially available flat panel digital detectors have been described. See, e.g., U.S. Pat. No. 6,987,831 B2 to Ning; and John Boone et al., “Computed Tomography for Imaging the Breast,” Journal Mammary Gland Biol Neoplasia, 11(2) at 103-111, (April, 2006). These systems acquire a series of cone beam views as the flat panel and x-ray source rotate around the breast and the images are reconstructed into three-dimensional images using cone beam CT algorithms. The flat panels provide an intrinsic pixed size of about 100-200 microns or spatial resolution of 2.5-5 lp/mm. X-ray scatter provides design challenges as each view is essentially a digital mammogram of the entire uncompressed breast and using conventional static or moving grids to reduce scatter increases image processing complexities. In addition, the kilovoltage as described for these systems ranges up to 80 kVp which will produce images of lower contrast than conventional digital mammography systems which typically use kilovoltage in the range of 30-40 kVp.